1. Technical Field of the Invention
The present invention relates to a method for reaction control coating to enhance oxidation resistance of an Ni-base superalloy while controlling the formation of a secondary reaction zone (SRZ).
2. Description of the Related Art
In order to improve the thrust and efficiency of an aircraft engine, high temperature materials constituting it are required to have improved high temperature properties and increased strength. In particular, an Ni-base superalloy used for turbine blades is excellent in high temperature strength, high temperature ductility and oxidation resistance so that the improvement of its properties greatly contributes to the improvement in the performance of the engine. A further improvement in the high temperature properties of the Ni-base superalloy has been made to respond to an increase in the temperature of a turbine inlet or reduction in the amount of cooling air.
A casting method of an Ni-base superalloy was conventional casting, but directional solidification casting has been developed, and then, single crystal casting has been developed. In particular, a single crystal superalloy is added with a heavy element to reinforce its γ′ precipitate phase or solid solution. The development of the single crystal superalloy has resulted in a first generation superalloy (free of Re), a second generation superalloy (Re content: about 3 wt. %) and a third generation superalloy (Re content: from 5 to 6 wt. %). With an advance in the development, an Re content of the superalloy increases. Table. 1 shows single crystal superalloys typical of each of the first generation to the third generation and their compositions.
TABLE 1TYPICAL Ni BASE SC SUPERALLOYS AND CHEMICAL COMPOSITIONS THEREOFELEMENT (wt %)MATERIALAlTiTaNbMoWReYHfCrCoRuNiFIRSTPWA14805.01.512.0—4.08.0———10.05.0—remainderGENERATIONCMSX-26.01.06.0—1.08.0———8.05.0—remainderSCRene′N43.74.24.00.52.06.0———9.08.0—remainderSECONDPWA14845.6—9.0—2.06.03.0—0.105.010.0—remainderGENERATIONCMSX-45.61.06.5—0.66.03.0—0.106.510.0—remainderSCRene′N56.2—6.5—1.55.03.00.010.157.07.5—remainderTHIRDCMSX-105.70.28.00.10.45.06.0—0.032.03.0—remainderGENERATIONRene′N66.0—7.00.31.06.05.00.010.204.013.0—remainderSCTMS-756.0—6.0—2.06.05.0—0.103.012.0—remainderTMS-1216.0—6.0—3.06.05.0—0.103.06.0—remainder
The single crystal superalloys of the third generation have the highest temperature capability and have been applied for turbine blades of latest aircraft engines. These superalloys however have problems that as illustrated in FIG. 1A, a needle-like harmful phase called “TCP phase” precipitates after long exposure at high temperatures and their strength decreases with an increase in this TCP phase.
The present inventors and others have already developed a fourth generation single crystal superalloy TMS-138 having, as a result of suppressing TCP formation by the addition of Ru, improved composition stability even after long exposure at high temperatures. FIG. 1B illustrates the microstructure of TMS-138 after creep rupture test. This drawing suggests that the formation of a TCP phase is suppressed. It has been confirmed that TMS-138 is particularly excellent in creep temperature capability, high cycle fatigue strength and low cycle fatigue strength. Table 2 shows the composition of the single crystal superalloy TMS-138.
TABLE 2CHEMICAL COMPOSITION OF TMS-138ELEMENT (wt %)MATERIALAlTiTaNbMoWReYHfCrCoRuNiTMS-1385.9—5.6—2.95.94.9—0.102.95.92.0REMAINDER
The single crystal superalloy described above is notified in Patent Document 1 and Non-patent Document 1, while the TCP phase and SRZ are notified in Patent Documents 2 and 3 and Non-patent Document 2.
In “DIFFUSION BARRIER LAYER” according to Patent Document 2, diffusion barrier coating is applied to an Ni base single crystal alloy (SC), and by aluminum diffusion coating is applied so that the coating layer can have improved oxidation resistance.
In “A method of aluminising a superalloy” according to Patent Document 3, a TCP phase or SRZ which tends to form on the interface between aluminum diffusion coating and SC is modified by a barrier layer.
[Patent Document 1]
Japanese Laid-Open Patent Publication No. 131163/1999, “Ni base single crystal alloy and manufacturing method thereof”
[Patent Document 2]
U.S. Pat. No. 6,306,524
[Patent Document 3]
European Patent Application No. 0821076
[Non-patent Document 1]
Yasuhiro Aoki, et al., “Present situation and problems in development of turbine blade materials for aircraft engine”, Research Report of Heat-resistant Metal Material 123 Committee, Vo. 43, No. 3
[Non-patent Document 2]
W. S. Walston, et al., “A NEW TYPE OF MICROSTRUCTURAL INSTABILITY IN SUPERALLOYS-SRZ”, Superalloys, 1996
Oxidation resistant coating must be applied to the surface of a turbine blade in order to prevent its high-temperature oxidation. Aluminum diffusion coating has been conventionally applied for this purpose. As a result of oxidation test and rupture test by using, as a test material, the above-described single crystal superalloy (TMS-138) to which aluminum diffusion coating has been applied, the coating causes uniform formation of SRZ as illustrated in FIGS. 2A and 2B, and drastically reduces the creep rupture time as illustrated in FIG. 4.